Method for producing calcium borates

ABSTRACT

A method of precipitating calcium borates from an aqueous mixture of calcium chloride and borax is disclosed. The yield of calcium borate precipitates can be increased by increasing the concentration of borax. Preferably, an increase in the calcium concentration also is employed. In general, it was found that properties of compacted glass batch produced from these calcium borates compare favorably with those compacted from standard glass batches.

TECHNICAL FIELD

This invention relates to a process for producing calcium borates havingparticularly useful properties. Among these properties are a relativelyhigh percentage of B₂ O₃, a relatively low percentage or completeabsence of Na₂ O and an unusually low water content.

BACKGROUND ART

Many industrial processes, of which glass making is illustrative,require appreciable quantities of B₂ O₃ as an ingredient in acomposition that is to be melted at relatively high temperatures in afurnace. The B₂ O₃ may be supplied by many different borate compounds.The following is a list of some of the more readily available boratecompounds:

    ______________________________________                                        Mineral or Chemical Name                                                                       Chemical Formula                                             ______________________________________                                        Boric acid       H.sub.3 BO.sub.3                                             Anhydrous boric acid                                                                           B.sub.2 O.sub.3                                              Anhydrous borax  Na.sub.2 O . 2B.sub.2 O.sub.3                                5 Mol borax      Na.sub.2 O . 2B.sub.2 O.sub.3 . 5H.sub.2 O                   Borax            Na.sub.2 O . 2B.sub.2 O.sub.3 . 10H.sub.2 O                  Dehydrated Rasorite                                                                            Na.sub.2 O . 2B.sub.2 O.sub.3                                Probertite       Na.sub.2 O . 2CaO . 5B.sub.2 O.sub.3 . 10H.sub.2 O           Ulexite          Na.sub.2 2CaO . 5B.sub.2 O.sub.3 . 16H.sub.2 O               Colemanite       2CaO . 3B.sub.2 O.sub.3 . 5H.sub.2 O                         Calcined colemanite                                                                            2CaO . 3B.sub.2 O.sub.3 . H.sub.2 O                          Sodium Perborate NaBO.sub.2 . H.sub.2 O.sub.2 . 3H.sub.2 O                    ______________________________________                                    

Because of the disadvantages of relatively large amounts of water orsoda in many of these compounds, a large potential demand for purecalcium borates exists. Ulexite, colemanite and especially calcinedcolemanite are desirable for glass making processes particularly wherecompacting glass batch is employed.

DISCLOSURE OF INVENTION

According to this invention, calcium borates are precipitated from anaqueous mixture of calcium chloride and borax.

This method of precipitating calcium borate from solutions of borax andcalcium chloride is becoming more desirable with the increasing use ofcompacting glass batch in the glass industry. Sources of boron otherthan ulexite and calcined colemanite now used in compacting glass batchwill be necessary in order to avoid dependence upon a single boronsource. Borax, boric acid and anhydrous boric acid (or boric oxide) areunacceptable in some compacting systems because they dissolve in wateror decrepitate when compacted and preheated. In other cases, the cost ofthe calcined product is prohibitive.

Both borax and calcium chloride are very soluble in water. Whensolutions of the two are mixed, a voluminous, finely divided whiteprecipitate is formed. The precipitate is somewhat difficult to filter,being so fine, but becomes less so upon standing or on heating. Theproduct is amorphous and contains about 50% water.

I have discovered a way of increasing the yield of calcium borate. Mydiscovery lies in increasing the concentration of borax in the originalsolution. Accompanied by an increase in the concentration of calcium, anincrease in the yield of precipitation is achieved. As will bedemonstrated by the following examples, the yield of precipitate levelsincreases to a particular point where higher concentrations of boraxfail to give substantially higher yields of precipitate.

The reaction of borax with the calcium chloride in water solutionaccording to this invention is as follows: ##STR1##

If a great excess of calcium chloride is added to the solution in anattempt to precipitate as much borate as possible, the originalprecipitate redissolves. This effect limits the amount of original boraxwhich can be converted into calcium borate by the process. If too muchcalcium chloride is added, the calcium borate goes back into thesolution; if too little is added, not all the borax is precipitated.Accordingly, my invention lies in the discovery of the criticalconcentrations of materials employed to yield the maximum amount ofcalcium borate.

Other halides that are highly soluble in water can be employed in thisinvention. Calcium iodide and calcium bromide which are highly solublein water, are included within the scope of this invention.

Calcium fluoride, on the other hand, is very insoluble in water andwould not be included. Other halide salts which are included are thoseof aluminum, magnesium, barium and the like.

Generally, the calcium chloride (or halide) calculated as CaO (or thecorresponding oxide) and the borax calculated as B₂ O₃ are present inthe initial amounts having a molar ratio of CaO/B₂ O₃ ranging from 0.2to 0.6. Preferably, the ratio of CaO/B₂ O₃ ranges from 0.3 to 0.5.

In general, it was found that properties of compacted glass batchproduced from these calcium borates compare favorably with thosecompacted from standard glass batches. In order to be useful for glassbatch compacting, the calcium borate must first be dried at about 100°C. for an hour to remove loosely bound water. The small amount ofremaining water will not be expected to affect the compacting process.

In order to further investigate this reaction and to optimize a yield, amethod of titration was developed. To a solution of borax of knownconcentration, an increment of a concentated calcium chloride solutionis added, which causes a calcium borate to precipitate. A small amountof the solution containing no precipitate is extracted and analyzed forboron. In this way, the remaining borate in solution is measured at eachstep, from which one may compute the amount in the precipitate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are titration curves.

FIG. 3 is a solubility curve.

In FIGS. 1 and 2, the circles are the measurements and straight linesare drawn between them. All amounts are molar quantities and are relatedto the initial borax expressed as B₂ O₃. In addition, the calcium addedis expressed as CaO. The titration in FIG. 1 was done at an initialborax concentration of 0.1 M Na₂ B₄ O₇. The tiltration in FIG. 2 wasdone at an initial borax concentration of 0.5 M Na₂ B₄ O₇. Thesolubility of calcium borate in FIG. 3 was measured as a function oftemperature.

BEST MODE OF CARRYING OUT INVENTION

As CaCl₂ is added to the borax solution, the amount of borate insolution decreases sharply and then levels off. As calcium is added tothe solution, at first nearly all of it is precipitated as calciumborate. Therefore, the initial slope of the titration curve gives thecomposition of the first precipitate. From FIG. 1, the slanted straightline shows that CaO and B₂ O₃ are precipitated at the ratio of 1:2, or acomposition of CaB₄ O₇ (or CaO.2B₂ O₃). Past a certain point, however,no borax is precipitated. The maximum yield of precipitate obtained fromthe amount of B₂ O₃ left in solution is 70% (1-0.3) in the case ofFIG. 1. This yield occurs at a ratio of added CaO to initial B₂ O₃ ofabout 0.35 and the product precipitated is CaB₄ O₇.

The yield of borate may be increased if higher concentrations of boraxare used. The titration curve in FIG. 2 is the same as that in FIG. 1,but done at an initial borax concentration of 0.5 M Na₂ B₄ O₇. Thiscurve also shows an initial slope approximately corresponding to apreipitate of CaB₄ O₇. The curve levels off at a precipitate yield ofabout 78% (1.0-0.22). Somewhat higher concentrations of borax were triedbut failed to give substantially higher yields than this.

According to this invention, the best yield of borates is given by aninitial concentration of 0.5 M borax to which is added an amount ofcalcium chloride solution to give an added CaO/initial B₂ O₃ ratio of0.5.

For the intended use of the calcium borate in compacting glass batch,the solubility in water is of interest. It is desirable that thesolublity not increase significantly with temperature since thetemperature changes during compacting and predrying of glass batch wouldlikely lead to an undesirable dissolution and reprecipitation cycle. Thefollowing results show that all of the calcium borates are virtuallyinsoluble in water and that their solubility is nearly independent oftemperature.

The solubility in pure water of a series of calcium borates was measuredat various tempertures from 20° C. to 70° C. using the method given inExample IV. Measurements were made not only on the calcium boratesproduced by the precipitation of this invention, but also on a series ofcalcium borates produced by other means.

The solubility of calcium borates precipitated from borax and calciumchloride is shown in FIG. 3 as a function of temperature. A straightline was drawn through the data points. From the data it can be seenthat the solubility of the calcium borates is very low and is roughlyconstant over the entire temperature range. Accordingly, the calciumborates would be suitable for compacting glass batch from a solubilitystandpoint.

Methods are known in the art for glass manufacturing whereinglass-forming, batch ingredients are compacted into agglomerates andthese agglomerates then are heated in a chamber by a direct contact withflue gases from a glass melting furnace so as to produce free-flowingagglomerates which are then conveyed and discharged to the glass meltingfurnace. These agglomerates are composite, integral, self-supportingmasses consisting essentially of all the substantial batch materials andmay take the form of balls, extrusions, discs, briquettes, and pellets.The pellets are discharged to a vertical bed contained within a chamberand furnace flue gases pass, in direct contact with and countercurrentlyto, downwardly moving pellets of the bed to dry and preheat them. Theprocess is carried out at a temperature and for a time insufficient tofuse or melt substantial amounts of the agglomerates. The agglomeratesusually will start to fuse together when their temperature reaches 1,400or 1,500° F., so contact is discontinued at or below these temperatures.

EXAMPLE I Procedure for the determination of calcium

(1) Dissolve the sample in distilled water or in a few ml of dilutehydrochloric acid if necessary.

(2) Dilute to 50 ml; add 1 ml of buffer solution and 4 drops ofindicator solution.

(3) Titrate with a standardized EDTA solution to a red to blue clorchange.

The buffer solution mentioned here is prepared by dissolving about 7grams of NH4Cl in 60 ml of concentrated NH4OH solution and diluting itin 100 ml of distilled water.

The EDTA solution is made by dissolving 18 grams of the disodium salt ofethylenediaminetetraacetic acid in somewhat less than 1000 ml ofdistilled water. Then 0.3 grams of MgCl₂.6H₂ O are dissolved in it andthe solution is made up to 1000 ml.

The EDTA solution is standardized by dissolving accurately weighed 0.2gram samples of dry CaCO₃ in a small amount of HCl solution. They arethen titrated with the EDTA solution following the above procedurestarting at step (2).

EXAMPLE II Procedure for determination of boron

This procedure applies to the determination of boron in borate solutionsor in borates which are soluble in water or in dilute acid. If theborate is already in solution, proceed to step (3).

(1) Accurately weigh a 0.2 gram sample of the borate to be determined.

(2) Add 2 ml of distilled water and 2 ml of 0.1 M HCl solution. Heat todissolve. If it fails to dissolve, dilute to 100 ml and heat. In no caseshould the solution come to a boil.

(3) Dilute the solution to 100 ml if not already and insert the pH meterprobes.

(4) Titrate with 0.1 M HCl to the sharp end point at pH 5.4. If the endpoint is overshot, titrate back with 0.1 M NaOH. The amounts of acid andbase need not be recorded.

(5) Add several grams of mannitol and titrate with standardized 0.1 MNaOH to pH 6.8.

(6) When 6.8 is reached, add more mannitol, which usually causes the pHto drop.

(7) Continue to titrate to pH 6.8.

(8) Repeat steps (6) and (7) until the addition of mannitol does notchange the pH.

(9) Record the volume of 0.1 M NaOH used in steps (5) through (8). Twomoles of NaOH are equivalent to one mole of B₂ O₃ in the originalsample.

The 0.1 M HCl and NaOH are conveniently standardized by titrating anaccurately weighed sample of dry Na₂ CO₃ with the HCl and then bytitrating the HCl and the NaOH with a pH meter.

EXAMPLE III Method of titration of borate solutions with calciumchloride

(1) Make up 100 ml of the required concentration of borax (orneutralized boric acid) in a 250 ml beaker.

(2) Extract exactly 1 ml (2 ml if under 0.1 M borax) by pipette andanalyze this sample for boron by the method in Example II, starting atstep (3).

(3) Add a measured increment of standardized CaCl₂ solution* by pipetteor burette and stir.

(4) Fold a piece of filter paper into a cone and immerse the point aboutone inch below the surface of the solution.

(5) In a few minutes enough solution will filter through to enable 1 mlto be extracted by pipette.

(6) Analyze this sample for boron according to Example II or for calciumby Example I, starting at step (2).

(7) Push the filter paper to the bottom of the beaker and repeat steps(3) to (7) as required.

(8) Calculate the number of moles B₂ O₃ (or CaO) left in the solutionfrom the amount in the 1 ml sample and from the total volume of thesolution from which it was taken. Note that the solution volume changeswith the addition of CaCl₂ and removal of 1 ml samples.

EXAMPLE IV Procedure for determining the solubility of a calcium borate

This method applies to the determination of the water solubility ofslightly soluble or moderately soluble calcium compounds as long as thecalcium content of the compound is known. More precisely, the methodmeasures the amount of soluble calcium released by a dissolved borate.This quantity is known to be the property of interest in the productionof agglomerates.

(1) With a constant temperature bath, bring about 200 ml of distilledwater to the highest temperature at which the solubility is to bemeasured.

(2) Dissolve as much calcium borate as possible, leaving some excesslying in the beaker.

(3) When the solution has reached equilibrium (about 2 hours), extractexactly 10 ml by pipette and analyze for calcium by the method ofExample I, starting at step (2).

(4) Lower the bath temperature to the next lower value desired.

(5) Repeat steps (3) and (4) as required to cover the temperature range.

The solubility of the calcium borate at each temperature may becalculated from the amount of calcium in the 10 ml sample.

I claim:
 1. A method for producing calcium borates by precipitation fromsolution comprising an initial step of forming an aqueous solution ofborax and calcium chloride wherein the calcium chloride calculated asCaO and the borax calculated as B₂ O₃ are presented in initial amountshaving a molar ratio of CaO/B₂ O₃ ranging from 0.2 to 0.6.
 2. A methodaccording to claim 1 wherein the ratio of CaO/B₂ O₃ ranges from 0.3 to0.5.
 3. A method according to claim 1 wherein the borax has an initialconcentration ranging from 0.1 to 0.5 mole.
 4. A method according toclaim 3 wherein the borax had an initial concentration of 0.1 M and theratio of CaO/B₂ O₃ is 0.35.
 5. A method according to claim 3 wherein theborax had an initial concentration of is 0.5 M and the ratio of CaO/B₂O₃ is 0.5.
 6. A method for producing calcium borates by precipitationfrom solution comprising an initial step of forming an aqueous solutionof borax oxide and a halide salt wherein the halide salt calculated asan oxide and the borax calculated as B₂ O₃ are present in initialamounts having a ratio by weight of halide salt to B₂ O₃ ranging from0.2 to 0.6.
 7. A method according to claim 6 wherein the borax has aninitial concentration ranging from 0.1 to 0.5 M.
 8. A method accordingto claim 1 wherein the calcium borates are subsequently dried at about100° C. for an hour.
 9. A method according to claim 8 wherein thecalcium borates are subsequently heated to over 250° C.
 10. A method forpreparing a calcium borate-containing glass fiber forming glass batchcomprising introducing calcium borate-containing batch ingredients intoa compacting zone, compacting the ingredients with sufficient water toproduce agglomerates containing 5% to 20% by weight of water, andheating the agglomerates to a temperature and for a time insufficient tomelt or fuse the agglomerates, wherein the calcium borate is prepared byforming an aqueous solution of borax and calcium chloride wherein thecalcium chloride calculated as CaO and the borax calculated as B₂ O₃ arepresent in initial amounts having a molar ratio of CaO/B₂ O₃ rangingfrom 0.2 to 0.6.
 11. A process according to claim 10 including thesubsequent step of charging the agglomerated glass batch to a glassmelting furnace.